There are many metal finishing processes, e.g., plating, etching, pickling, brightening, etc., where either salt impurities or metal values build up continuously in the treatment solutions to undesirably high levels during use, ultimately requiring discharge or regeneration of said solutions. Since the spent solutions usually contain valuable constituents in the form of considerable residual quantities of reactants and/or metal values, regeneration of the solutions for removal of the impurities or metal values, e.g., by crystallization, is desirable. As is well known, crystallization of a chemical compound such as a salt from a solution is caused by a temperature change in the solution and begins whenever the solubility of the salt is exceeded in view of this temperature change. Crystal formation on cooling normally occurs on the lowest temperature surface, such as on cooling coils, heat exchangers, tank walls etc. Similarly in those cases where the solubility of a compound decreases with increasing temperature, the crystal formation occurs on the hottest surfaces, i.e, the heat transferring surfaces. Crystal removal under these conditions requres an intense labor effort. In large production systems for the manufacture of crystalline chemicals, evaporation and spray crystallization with drying is most often used to avoid contact between heat exchange surfaces and the pregnant liquor. Other systems use a continuous scraper mechanism to keep the heat exchange surfaces cleaned. The crystal product is usually recovered as relatively dilute slurrys and need to be concentrated in equipment such as centrifuges and hydrocyclones. However, the use of these types of sophisticated and complex equipment for regeneration of metal processing solutions is not feasible from either a practical or economical standpoint for the average metal processor. Specifically, the quantities of the materials to be crystallized are usually very low, and the total inventory of spent solutions are relatively small, generally not exceeding 800 gallons. Typically, the amount of impurities or metal salts to be recovered by crystallization ranges between a few ounces to a maximum of 30 pounds per hour. Also the spent solutions to be regenerated are often highly corrosive requiring that all equipment, which is in contact with the solution be constructed of expensive corrosion resistant metals or metal alloys.
The present practice in metal finishing is therefore to cool (or heat) the solution to be regenerated by inserting cooling (heating) coils into a spent solution. As the temperature is changed, crystallization occurs on the coils which are then removed from the solution to be regenerated, either at the time the desired crystallization temperature has been reached and the crystallization is completed, or when the heat exchange surfaces appear insulated due to crystal growth. The crystals are removed from the coils by scraping or by redissolution in water. Because of the inherent detrimental change in heat transfer rate as the crystals build up on the coils, and ultimate fouling of the heat exchange surfaces, the above described method does not lend itself to a continuous regeneration process. Furthermore, the presently used method requires considerable manual handling, which is both costly and hazardous, since the solutions to be treated are generally quite noxious. Another disadvantage of the aforementioned method is that it provides no control over the crystal size distribution.
It is therefore an object of this invention to provide a novel apparatus and process for the continuos removal of a salt from a solution by crystallization.
Another object is to provide an apparatus and process for the continuous direct recovery of a concentrated salt crystal slurry.
Still another object is to provide an inexpensive and efficient apparatus and process for the continuous regeneration of corrosive metal processing soltions.
A further object is to provide an apparatus and continuous process for the recovery of crystals of a controlled particle size distribution.
Other objects and advantages will become apparent from a reading of the detailed description of the invention.